Add SPDX identifier. Fix #1387

src/license.h

@@ -1,5 +1,5 @@
 // Copyright (c) 2004-2013 Sergey Lyubka
-// Copyright (c) 2013-2021 Cesanta Software Limited
+// Copyright (c) 2013-2022 Cesanta Software Limited
 // All rights reserved
 //
 // This software is dual-licensed: you can redistribute it and/or modify
@@ -14,3 +14,5 @@
 //
 // Alternatively, you can license this software under a commercial
 // license, as set out in https://www.mongoose.ws/licensing/
+//
+// SPDX-License-Identifier: GPL-2.0 or commercial